It is known to produce methyl cellulose and mixed ethers thereof discontinuously in reaction vessels where first alkali cellulose is obtained by steeping cellulose fibers or chips in aqueous alkali metal hydroxide solution and then reacting the alkali cellulose with excess methyl chloride after squeezing and coarse crushing. The reaction itself takes from about 2 to 3 hours according to this method, exclusive of the heating time, and thus one charge or production run takes a total of from about 4 to 5 hours.
Substantially shorter residence times can be obtained in continuous methods, such as is disclosed in German Published Application (DAS) No. 15 43 136. The alkali cellulose can be produced, just as in the discontinuous method, by dipping and mashing cellulose fibers or chips in aqueous sodium hydroxide solution and pressing the resulting pulp mixture or by mixing cellulose powder and aqueous sodium hydroxide solution. The reactor consists of a tube containing a screw worm. The alkali cellulose is reacted with an about 6-fold excess of methyl chloride at a temperature of 75.degree. to 85.degree. C. and a pressure of about 25 bar, and is advanced through the reaction chamber as it reacts. The heat of reaction can be eliminated by direct evaporation of excess methyl chloride. The reaction in the reaction mixture, which is primarily solid, is substantially isothermal.
A substantially continuous method is disclosed in German Published Application (DOS) No. 26 36 935. According to the method, a suspension of a finely-divided polysaccharide (cellulose), an aqueous alkali metal hydroxide solution, and at least one etherification agent is prepared in a suspension vessel under pressure. The suspension, to which an inert diluent can be added, is then introduced substantially continuously into a tubular reactor. This method also works with a 5- to 10-fold excess of methyl chloride. During the reaction the pressure is about 20 bar, while the reaction time is from 10 to 60 minutes. However, there is a great demand for mixed ethers of methyl cellulose, and, according to the state of the art, no such mixed ethers of methyl cellulose can be obtained in this continuous operation.